The PM will say that United Nations Security Council-mandated inspectors have probed previous attacks and decided Bashar Assad’s regime was responsible four times.

“We are confident in our own assessment that the Syrian regime was highly likely responsible for this attack and that its persistent pattern of behaviour meant that it was highly likely to continue using chemical weapons,” she will add.

“Furthermore, there were clearly attempts to block any proper investigation, as we saw with the Russian veto at the UN earlier in the week.

She will say: “Let me be absolutely clear: we have acted because it is in our national interest to do so.

“It is in our national interest to prevent the further use of chemical weapons in Syria – and to uphold and defend the global consensus that these weapons should not be used.

“For we cannot allow the use of chemical weapons to become normalised – either within Syria, on the streets of the UK or elsewhere.”

The Prime Minister spent Saturday evening speaking to world leaders to explain why Britain had joined forces with France and the US and will insist the three nations are “not alone” in believing it was the “right thing to do”.

“There is broad based international support for the action we have taken,” she will say.

Germany, Italy, Canada, Australia and European Council President Donald Tusk have “all have expressed their support for the actions that Britain, France and America have taken”, the PM will add.

Four Royal Air Force Tornado GR4s joined the co-ordinated missile strikes at 2am on Saturday, launching Storm Shadow missiles at a base 15 miles west of Homs.

The United States is preparing to impose sanctions on Russia for “covering up” the actions of the Assad regime.